Shared via Pony Playground (https://playground.ponylang.io/?gist=af297d4d7583257416a4fcb8d732ae0f)

main.pony

actor Main
  
  new create(env: Env) =>
    env.out.print(fib(5).string())
    
  fun fib(n: U64): U64 =>
    match n
    | 0 => 0
    | 1 => 1
  else
    fib(n - 1) + fib(n - 2)
//    env.out.print(n.string())
  end